The invention is directed to a process and an apparatus for gas treatment, in particular for the purification of methane rich gas streams, such as gas obtained from the conversion of waste or other types of organic matter (“biogas”). More in particular, the present invention is directed to the production of purified biogas (such as landfill gas, digester gas), which finds use as an efficient energy source. Thus, the present invention is directed to a process and an apparatus to purify such methane rich gases to obtain a gas of a quality that is comparable to that of natural gas.
Various methods are known to produce biogas from organic matter, e.g. by the anaerobic digestion of organic waste (including manure, wastewater sludge, municipal solid waste, etc.). Apart from methane, these fermentation processes usually also produce together large quantities of CO2 (typically about 30-50 vol. % of the produced gas volume may be CO2) and smaller quantities of sulphides and compounds such as vinyl chloride as well as other impurities. The gas must be removed with mechanical work and then treated before it can be used as an energy source. The purified gas can be transported in cylinders, liquefied in cryogenic tanks, used for on-site power generation or piped to a nearby energy demand. The last two options appear to have the widest growth potential.
Methane gas is produced in large quantities in for example, landfill sites and certain sewage works. It is currently recovered in a few locations but the bulk of this potentially valuable energy reserve is lost. Exploitation of landfill gas, digester gas and other types of biogas has been limited particularly because no equipment is available for economically processing the gas into transportable form, which includes purification of the gas. Desirably the equipment itself should be transportable since, in many cases (e.g. landfill sites), the gas source will only produce viable quantities of gas for a limited time and the installation of permanent plant would not be justified.
Removing moist, H2S, SO2, halogens, siloxanes and/or other contaminants is essential to purifying landfill gas, digester gas and biogas for use as an efficient energy source. Although several well established technologies are used, the provision of a viable process for cleaning landfill gas, digester gas and biogas remains a problem. Various methods of gas treatment processes are already known for this purpose. These processes are focused on either the reduction of the moisture level content, CO2 extraction, H2S removal or the removal of one or more contaminants present in the gas mixture. The disadvantage of the known methods is that they do not to a sufficient extent permit the removal of many of the undesirable substances comprised in gases of the above-mentioned kind. Moreover, present processes are not entirely without disadvantages regarding the level of engine corrosion, wear, maintenance costs, generated process waste, disposal costs and composition of gas mixture as end-product. Finally, the prior art does not show how substances that are removed may be reused.
GB-A-1 565 615 describes a process for the separation of CO2 from methane wherein the mixture of the two compounds is cooled two form a gas/liquid mixture which is subsequently fractionated.
U.S. Pat. No. 4,681,612 describes a process for separating a landfill gas to produce a CO2 product stream and a fuel-grade methane product stream. The process involves cryogenic distillation and a membrane separation step to produce the fuel-grade methane stream.
In the art, also contaminants absorption processes, such as processes based on activated carbon filters, in which most contaminants are removed, are known. However, the regeneration costs and disposal costs for activated carbon are high. In addition carbon filters have a great affinity for moisture thus reducing adsorption efficiency. Furthermore, other absorption methods are applied such as molecular membrane filters, activated polymeric membrane filters and silica-gel. Yet, methods used still do not achieve quality level necessary for use as an efficient energy source.
Other absorption methods include contaminants elimination through organic dissolution. This process is in practice rather complicated because the contaminants targeted are highly volatile. Due to its chemical affinity for water or CO2, it is also possible to remove siloxanes by using condensation methods. In general however, lower elimination levels are achieved through this method.
The chemical make up of the landfill gas, digester gas and biogas differs from site to site. Some sites may have higher sulphur content, while others may have higher traces of siloxanes, and still others may have traces of heavy hydrocarbons, which can increase the risk of engine knocking when the gas is applied in a gas engine. As such, the gas treatment system has to be customized for each site depending on the levels of contaminants and engine requirements. Typical requirements for a biogas to be used in a gas engine are given in Table 1.
TABLE 1Minimum acceptable level of contaminants for gas enginesContaminantLevelRemarksHydrogen Sulphide<200 mg/Nm3 methaneEngine corrosionHalides (Chlorine,<100 mg/Nm3 methaneEngine corrosionFluorine)Ammonia<50 mg/Nm3 methane—Particulates (Dust)<3-10 μm—Hydrocarbons > C5<5 mg/Nm3 methaneIncrease engineknockingSiloxanes *)<2 mg/Nm3 methaneIncrease maintenanceReduced engineperformanceMoisture<80% RH at lowestAcid formationtemperature*) Depending on maintenance interval as contracted with gas engine manufacturer. Indicated value is average.
Of these contaminants siloxanes are the most aggressive and usually pose the biggest problems. Siloxanes break down into a white abrasive powder, which may damage equipment installed downstream (boilers, combustion engines, turbines, catalysts, or the like). Also, traces of siloxanes, hydrogen sulphide, halogens and other minerals have been reducing the performance and increasing maintenance of the generators. To the present inventor's best knowledge, to date no complete package is offered to remove large part of moist, H2S, SO2, halogens, siloxanes and other contaminants involved.
An object of the present invention is to provide a method which will permit the removal of several of the contaminating substances comprised in biogas. A further object is that the removal of those substances may take place in a simple manner so that the method is not dependent on complicated technical installations. Furthermore, the process of the invention should provide a clean gas having a level of contaminants that is sufficiently low to allow for problem-free application in for instance a gas engine. In other words, in accordance with the present invention the end-product should have a composition that provides a product that is close to natural gas quality. A further object is the process of the present invention should produce as little waste as possible and should be energy efficient.